LHCb spots CP violation in charmed mesons
Where did we come from?
That’s a really big question for anyone to answer. And if we want to answer such questions, we’re going to have to break it down into smaller questions, and then break the smaller questions further until we have something we know we can pin down.
One of the smaller questions we’ll need to answer to unravel the mystery of our origins is why the universe is made of matter and not antimatter. To be fair, this is still a pretty big question, so scientists have been looking for clues in the way fundamental particles work. After all, you really can’t get smaller than that.
On March 21, physicists announced that they’d observed one such particle display peculiar behaviour. The observation is one of the small things that need to fall in line to one day explain where all the antimatter went.
The universe is made entirely of matter today even though there were equal quantities of matter and antimatter at the moment of its birth.
The discovery was announced at the Rencontres de Moriond, an annual particle physics conference that happens in Italy, as well as at a special CERN seminar. CERN is the European lab for nuclear research that runs the Large Hadron Collider (LHC), the world’s largest physics experiment and where the physicists made their discovery.
The ‘peculiar behaviour’ is called CP violation, and is believed to be responsible for the universe losing all its antimatter as it evolved, before the first atoms formed.
‘CP’ stands for charge-parity. If our universe adheres to CP symmetry, then a particle replaced with its antiparticle and its spin replaced with its mirror-spin should behave the same way as the original particle. To rephrase Patrick Koppenburg, a member of the collaboration that made the discovery, “antimatter seen in a mirror should look like matter”.
However, our universe violates CP symmetry. Antimatter seen in the mirror doesn’t look like matter, and this aberration could have helped wipe out the universe’s supply of antimatter.
CP violation has previously been observed in two kinds of mesons. Mesons are particles made of one quark and one antiquark of different types.
In all, there are six kinds of quarks – and six kinds of anti-quarks: up, down, top, bottom, charm, strange. And they combine to form dozens of different kinds of mesons. For example, a kaon is a meson made of one strange quark and one up/down antiquark; a B meson is a meson made of one bottom antiquark and one up/down/strange/charm quark. And physicists have observed kaons and B mesons violating CP symmetry in the 1960s and in 2001, respectively.
On March 21, physicists working with a detector called the LHCb, at the LHC, announced that a third particle had joined this group: the D0 meson, discovered in the 1970s. Each D0 meson is made of a charm quark and an up antiquark. This is the first time a particle comprising the charm quark has showed signs of violating CP symmetry.
But even though we now have three instances of CP violation, the matter-antimatter problem isn’t considered solved. This is because the instances aren’t enough by themselves to explain why all of the antimatter has gone away. We need other, perhaps stronger ‘sources’.
For example, CP symmetry violation has thus far been observed only in particles containing quarks and/or antiquarks. We also need to find proof of this violation in the ‘lepton sector’ – i.e. observe electrons and neutrinos violating CP symmetry – and in interactions involving the strong nuclear force. And this is just in the Standard Model of particle physics, which is a set of rules physicists use to understand the currently known elementary particles.
Key to uncovering all of these is figuring out why the violation happens in the first place in the particles already in the dock.
The crime at the heart of the CP symmetry violation is committed by a fundamental force called the weak nuclear force. This force is famous for causing radioactivity in heavy atoms like those of uranium and plutonium. The same force also preferentially interacts with left-handed quarks, and ignores right-handed quarks.
So in a series of reactions involving quarks and antiquarks, among other particles, the weak force ensures that processes that produce matter happen more often than those that produce antimatter. This way, there is a lot of matter still left over after some of it has combined with antimatter and turned into pure energy.
Matt Strassler, a theoretical physicist, called the discovery of CP violations in D0 mesons a “real coup” for the LHCb in a blog post, as well as that it was “consistent with expectations”.
The Standard Model of particle physics already predicts that these violations should occur in different particles. However, the predictions are somewhat approximate – not with as many decimal places as we’d like.
This is because, as mentioned earlier, there are three expected sources of CP violations: quark sector, lepton sector and interactions involving the strong nuclear force. So when a D0 meson violates CP symmetry, the extent of its violation has two contributions: some from the quark sector and some from the strong nuclear force, the force that holds quarks together. And calculations involving the strong nuclear force are extremely complicated, so physicists make approximations on the road to finding an answer.
As a result, we don’t know how well the extent of violation spotted by the LHCb and the extent of violation predicted by the Standard Model match up. If they’re close, then that’s okay; the discovery of CP violation in D0 mesons will have been something we already saw coming. But if it’s not close – i.e. if the extent of violation seen by the LHCb is greater than what the Standard Model predicts – then it becomes very, very interesting.
So far, the Standard Model has explained the behaviour of all known fundamental particles: leptons, quarks and bosons. But it doesn’t have answers to questions about why the particles’ properties are what they are, why there are six types of quarks, what dark matter is, etc. Many physicists expect there are more particles out there whose behaviour can help answer these questions. The physics of these particles is called ‘new physics’.
Long story short: we don’t know if the CP violation in D0 mesons is a sign of ‘new physics’ yet. If it is, it will then be a monumental result. But it’s not likely to be because the Standard Model is notoriously good at making accurate predictions.
But as Marco Gersabeck, a physics lecturer at the University of Manchester, wrote, “There’s every reason to be optimistic that physics will one day be able to explain why we are here at all.”
The Wire
March 22, 2019
